Layer-by-layer assembly of graphene oxide membranes via electrostatic interaction and eludication of water and solute transport mechanisms

ABSTRACT

A method for synthesizing a water purification membrane is presented. The method includes stacking a plurality of graphene oxide (GO) nanosheets to create the water purification membrane, the stacking involving layer-by-layer assembly of the plurality of GO nanosheets and forming a plurality of nanochannels between the plurality of GO nanosheets for allowing the flow of a fluid and for rejecting the flow of contaminants. The method further includes cross-linking the plurality of GO nanosheets by 1,3,5-benzenetricarbonyl trichloride on a polydopamine coated polysulfone support.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/905,219 filed Feb. 26, 2018, now U.S. Pat. No. 10,239,302, which is adivisional of, and claims the benefit of and priority to, U.S. patentapplication Ser. No. 14/658,990 filed Mar. 16, 2015, entitled“Layer-by-Layer Assembly of Graphene Oxide Membranes Via ElectrostaticInteraction and Elucidation of Water and Solute Transport Mechanisms”,by Mi Baoxia et al., now U.S. Pat. No. 9,902,141, which claims thebenefit of and priority to U.S. Provisional Patent Application Ser. No.61/953,418 filed Mar. 14, 2014, entitled “Layer-by-Layer Assembly ofGraphene Oxide Membranes for Separation” by Mi Baoxia et al., the entirecontents of each of which are incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with U.S. government support under CBET1158601and CBET11544572 awarded by the National Science Foundation (NSF). TheU.S. government has certain rights in this invention.

BACKGROUND Technical Field

The present disclosure relates to water treatment processes usingnanomaterials in membrane synthesis and surface modification. Moreparticularly, the present disclosure relates to systems and methods forusing a layer-by-layer (LbL) assembly of graphene oxide (GO) nanosheetsvia bonding techniques, such as covalent bonding and electrostaticinteraction.

Description of Related Art

Dwindling water resources and increasing water consumption have forcedresearchers to consider new advanced water treatment technologies thatcan provide a safe water supply in a more efficient, environmentallysustainable manner. Nanofiltration (NF), reverse osmosis (RO), andforward osmosis (FO) membrane processes are among the most effectivestrategies to achieve high removal of both traditional and emergingcontaminants from water. All these processes require the use ofsemi-permeable membranes, the market of which has been dominated fordecades by thin film composite (TFC) polyamide membranes due to theirsalient advantages, such as good separation capability and wide pHtolerance. Despite their advantages, TFC membranes face technicallimitations regarding, for example, chlorine resistance, foulingresistance, and energy efficiency. It is also a challenge to make TFCmembranes with thinner, more hydrophilic and more porous support layers,which are crucial for high-performance membranes.

The recently emerging graphene-based nanomaterials have exhibitedinteresting properties, such as adsorption of metal and organic dyes,antimicrobial capability, and photocatalytic degradation of organicmolecules. In particular, graphene oxide (GO) nanosheets offer anextraordinary potential for making functional nanocomposite materialswith high chemical stability, strong hydrophilicity, and excellentantifouling properties. In recent years, nanomaterials have beenextensively used in membrane synthesis and surface modification toimprove membrane performance (e.g., flux, antibacterial property,fouling resistance, photocatalytic property) or to optimize theoperation of membrane processes (e.g., energy consumption, maintenancerequirement). Because the use of these nanomaterials often relies onexpensive materials, costly facilities, and highly complex synthesis, itbecomes very desirable to make high-performance water separationmembranes using low-cost raw materials and facile yet scalable synthesismethods.

As a derivative of graphene, GO nanosheets can be mass-produced viachemical oxidization and ultrasonic exfoliation of graphite. Hence, GOnanosheets bear hydroxyl, carboxyl, and epoxide functional groups on theplane of carbon atoms and thus have a more polar, hydrophilic character.A GO nanosheet is single-atom-thick with lateral dimensions as high astens of micrometers, making it highly stackable. Stacked GO nanosheetsmade via a simple solution filtration method can exhibit excellentmechanical strength in dry conditions.

The concept of using graphene-based nanomaterials to make waterseparation membranes was first examined using molecular simulations.Nanopores are “punched” through a super-strong graphene monolayer sothat water can permeate through the single-atom-thick membrane whileother substances are selectively rejected. By controlling pore sizes andfunctional groups on graphene, such a monolayer graphene membrane couldbe useful for desalination, with a water permeability of severalmagnitudes higher than that of current reverse osmosis (RO) membranes.An experimental study was recently reported to create such porousgraphene membranes and test their selectivity for gas separation.Despite these simulation and experimental efforts, at presentsignificant technical difficulties exist in making such monolayergraphene membrane for real-world water separation. For example, it isstill impractical to prepare a large area of monolayer graphene, and itis extremely challenging to obtain high-density nanopores withcontrollable, relatively uniform sizes on a graphene sheet.

An alternative approach is to synthesize a water separation membranewith stacked GO nanosheets. The spacing between the neighboring GOnanosheets creates 2D nanochannels that may allow water to pass throughwhile rejecting unwanted solutes. Water can flow at an extremely highspeed in such planar graphene nanochannels. A recent experimental studyhas revealed unimpeded permeation of water vapor (at a rate 10¹⁰ timesfaster than helium) through a stacked GO membrane, a phenomenon thatcould be attributed to a nearly frictionless flow of a monolayer ofwater through 2D capillaries formed by closely spaced GO nanosheets.Although tested for gas/vapor separation only, stacked GO nanosheetshold great potential for making highly permeable water separationmembranes to remove various types of contaminants.

Stacked GO membranes reported so far in the literature, however, aremade simply via solution filtration. Hence, they are not suitable forwater separation applications due to the lack of necessary bondingbetween stacked GO nanosheets. This is because GO nanosheets areextremely hydrophilic and thus these membranes tend to easily dispersein water. Even if some performance data could be collected throughextremely careful handling of the membrane made with unbonded GOnanosheets, such a GO membrane unfortunately does not survive thecross-flow testing conditions, which are typical in real-world membraneoperation. Therefore, these unbonded GO membranes should not beconsidered or used as water separation membranes.

After a GO membrane has been synthesized, the oxygen-containingfunctional groups on GO provide convenient sites for furtherfunctionalization to adjust various properties (e.g., charges,interlayer spacing, specific interactions with water contaminants) of GOnanosheets. For example, GO can be covalently functionalized by aminegroups to modify charges, sulfonic groups to make ion/proton-exchangemembranes, and polymers to enhance biocompatibility. GO can also benon-covalently bonded with various monomers, polymers, and evennanoparticles to adjust mechanical, thermal, and chemical properties.These exceptional properties of GO provide for flexibility to optimizenot only membrane permeability by varying the size and morphology of thefunctional groups (thus adjusting GO interlayer spacing) but alsomembrane selectivity by adjusting charge, charge density, and specificinteractions with water contaminants.

To date, however, synthesis of a water separation membrane by the properbonding and optimization of stacked GO nanosheets has not been reported.

SUMMARY

Embodiments of the present disclosure are described in detail withreference to the drawing figures wherein like reference numeralsidentify similar or identical elements.

An aspect of the present disclosure provides a method for synthesizing awater purification membrane, the method including stacking a pluralityof graphene oxide (GO) nanosheets to create the water purificationmembrane, the stacking involving layer-by-layer assembly of theplurality of GO nanosheets and forming a plurality of nanochannelsbetween the plurality of GO nanosheets for allowing the flow of a fluidand for rejecting the flow of contaminants.

In one aspect, the method further includes cross-linking the pluralityof GO nanosheets by 1,3,5-benzenetricarbonyl trichloride on apolysulfone support.

In another aspect, the polysulfone support is a polydopamine coatedpolysulfone support.

In yet another aspect, the plurality of GO nanosheets are negativelycharged over a wide pH range.

In one aspect, the method further includes covalently bonding theplurality of GO nanosheets via cross-linkers. The cross-linkers may bemonomers and polymers.

In another aspect, the method includes electrostatically bonding theplurality of GO nanosheets. A structure, a charge, and a functionalityof the plurality of GO nanosheets may be tuned by usingpolyelectrolytes.

In yet another aspect, the lateral sizes of the plurality of GOnanosheets vary between 100 and 5000 nm, whereas a thickness of theplurality of GO nanosheets varies between 1 and 2 nm.

In another aspect, the stacking results in at least a portion of theplurality of GO nanosheets being arranged in a non-overlapping manner.

Another aspect of the present disclosure provides a method for creatinga water separation membrane, the method including depositing a pluralityof graphene oxide (GO) nanosheets via a layer-by-layer assembly andbonding the plurality of GO nanosheets with each other and with asupport substrate.

Certain embodiments of the present disclosure may include some, all, ornone of the above advantages and/or one or more other advantages readilyapparent to those skilled in the art from the drawings, descriptions,and claims included herein. Moreover, while specific advantages havebeen enumerated above, the various embodiments of the present disclosuremay include all, some, or none of the enumerated advantages and/or otheradvantages not specifically enumerated above.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure are described herein belowwith references to the drawings, wherein:

FIG. 1 illustrates the chemical composition of graphene oxide (GO), inaccordance with embodiments of the present disclosure;

FIG. 2 illustrates a layer-by-layer (LbL) assembly of GO nanosheets, inaccordance with embodiments of the present disclosure;

FIG. 3A illustrates a graph of a zeta potential measurement of the GOnanosheets of FIG. 2, in accordance with embodiments of the presentdisclosure;

FIG. 3B illustrates a graph of X-ray photoelectron spectroscopy (XPS)data related to the GO nanosheets of FIG. 2, in accordance withembodiments of the present disclosure;

FIG. 3C illustrates an atomic force microscopy (AFM) image of the GOnanosheets of FIG. 2, in accordance with embodiments of the presentdisclosure;

FIG. 3D illustrates a graph of AFM height profiles of the GO nanosheetsof FIG. 2, in accordance with embodiments of the present disclosure;

FIG. 4A illustrates a LbL assembly of a covalently bonded GO synthesisstrategy, in accordance with embodiments of the present disclosure;

FIG. 4B illustrates a LbL assembly of an electrostatically bonded GOsynthesis strategy, in accordance with embodiments of the presentdisclosure;

FIG. 5A illustrates a quartz crystal microbalance with dissipation(QCM-D) graph of the covalently bonded GO synthesis strategy of FIG. 4A,in accordance with embodiments of the present disclosure;

FIG. 5B illustrates a quartz crystal microbalance with dissipation(QCM-D) graph of the electrostatically bonded GO synthesis strategy ofFIG. 4B, in accordance with embodiments of the present disclosure;

FIG. 6A illustrates a graph of water flux of covalently bonded GOmembranes, in accordance with embodiments of the present disclosure;

FIG. 6B illustrates a graph of the rejection rate of a first chemicalcompound when covalently bonded GO membranes are used, in accordancewith embodiments of the present disclosure;

FIG. 6C illustrates a graph of the rejection rate of a second chemicalcompound when covalently bonded GO membranes, in accordance withembodiments of the present disclosure;

FIG. 7 illustrates a cross-sectional image of the polyacrylonitrile(PAN) support, with front and back views, in accordance with embodimentsof the present disclosure;

FIG. 8 illustrates a graph of the Fourier Transform InfraredSpectroscopy (FTIR) spectra of the GO membrane, in accordance withembodiments of the present disclosure;

FIG. 9 illustrates a graph of the elemental ratios of membrane supportsand GO membranes by XPS analysis, in accordance with embodiments of thepresent disclosure;

FIG. 10 illustrates a graph of charge density of membrane supports byQCM-D, in accordance with embodiments of the present disclosure;

FIG. 11 illustrates a graph of zeta potentials of GO and poly(allylaminehydrochloride) (PAH) at different pHs, in accordance with embodiments ofthe present disclosure;

FIG. 12 illustrates a schematic diagram of an LbL assembly of a GOmembrane by alternately soaking an hPAN support substrate in 1 g/L PAH(pH 4) solution and 1 g/L GO solution (pH 4), in accordance withembodiments of the present disclosure;

FIG. 13 illustrates SEM images of the GO membranes made of differentnumbers of GO-PAH bilayers, in accordance with embodiments of thepresent disclosure;

FIG. 14 illustrates a graph of cumulative masses of GO and PAH duringthe LbL assembly of a GO-PAH film on an hPAN-coated QCM-D sensor, inaccordance with embodiments of the present disclosure;

FIG. 15 illustrates a graph of pure water permeability under hydraulicpressure, in accordance with embodiments of the present disclosure;

FIG. 16 illustrates a graph of water flux in FO and PRO modes with 1Msucrose, in accordance with embodiments of the present disclosure;

FIG. 17 illustrates a graph of water flux in FO and PRO modes with 1MMgCl₂, in accordance with embodiments of the present disclosure;

FIG. 18 illustrates a graph of water flux in FO and PRO modes with 0.25M TSC, in accordance with embodiments of the present disclosure;

FIG. 19 illustrates a graph of sucrose permeation flux, in accordancewith embodiments of the present disclosure; and

FIG. 20 illustrates a graph of diffusion coefficients of draw solutesfor the 10-bilayer GO membrane, compared with those for water, inaccordance with embodiments of the present disclosure.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following disclosure that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles of the present disclosure described herein.

DETAILED DESCRIPTION

Although the present disclosure will be described in terms of specificembodiments, it will be readily apparent to those skilled in this artthat various modifications, rearrangements and substitutions may be madewithout departing from the spirit of the present disclosure. The scopeof the present disclosure is defined by the claims appended hereto.

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the exemplaryembodiments illustrated in the drawings, and specific language will beused to describe the same. It will nevertheless be understood that nolimitation of the scope of the present disclosure is thereby intended.Any alterations and further modifications of the inventive featuresillustrated herein, and any additional applications of the principles ofthe present disclosure as illustrated herein, which would occur to oneskilled in the relevant art and having possession of this disclosure,are to be considered within the scope of the present disclosure.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. The word “example” may be usedinterchangeably with the term “exemplary.”

The exemplary embodiments of the present disclosure present a novel typeof water purification membrane that was synthesized by layer-by-layer(LbL) assembly of negatively charged graphene oxide (GO) nanosheets onboth sides of a porous poly(acrylonitrile) support and interconnected bypositively charged poly(allylamine hydrochloride) (PAH) via, forexample, electrostatic interaction. Transport of water and selectedsolutes in the GO membrane was investigated in a pressurized system andalso in a forward osmosis (FO) and pressure retarded osmosis system, asdescribed in detail below.

Water permeability of the GO membrane was found to be about one order ofmagnitude higher than that of a commercial FO membrane, corresponding towater flowing in the GO channel at a speed of two orders of magnitudehigher than that predicted by the Poiseuille equation. The dominant pathfor water and solute transport was most likely formed by the clear space(˜1 nm) between layered GO nanosheets. Although the GO membrane mighthydrate in solutions of high ionic strength, it retained a tightstructure and exhibited high rejection and slow diffusion of solutes insolutions of low ionic strength. Hence, the GO membrane at the currentstage can be well suited for applications such as FO-based emergencywater supply systems using sugary draw solutions and water treatment notrequiring high ionic strength.

FIG. 1 illustrates the chemical composition 100 of graphene oxide, inaccordance with embodiments of the present disclosure.

The present disclosure presents an approach for the synthesis andsurface modification of water separation membranes by layer-by-layer(LbL) assembly of graphene oxide (GO) nanosheets. The GO membranes havehigh water permeability and improved selectivity for targetedcontaminants, thereby representing an alternative to current waterseparation membranes.

FIG. 2 illustrates a layer-by-layer assembly 200 of graphene oxidenanosheets, in accordance with embodiments of the present disclosure.

A procedure is presented to synthesize a water separation membrane usingGO nanosheets 210 such that water 220 can flow through the nanochannelsbetween GO layers 210 while unwanted solutes 230 are rejected by sizeexclusion and charge effects, as illustrated in FIG. 2. The GO membraneis made by a LbL deposition of GO nanosheets 210, which are cross-linkedby 1,3,5-benzenetricarbonyl trichloride on a polysulfone support 240having a polydopamine layer 242. The cross-links provide the stacked GOnanosheets 210 with the necessary stability to overcome their inherentdispensability in a water environment and also fine-tune the charges,functionality, and spacing of the GO nanosheets 210.

The membranes were synthesized with different numbers of GO layers todemonstrate their water separation performance. GO membrane flux rangedbetween 80 and 276 LMH/MPa, roughly 4-10 times higher than that of mostcommercial nano-filtration membranes. Although the GO membrane in thepresent development stage had a relatively low rejection (6-46%) ofmonovalent and divalent ions, it exhibited a moderate rejection (46-66%)of Methylene blue and a high rejection (93-95%) of Rhodamine-WT.

Thus, it was demonstrated that selective and permeable GO membranes canbe synthesized via an LbL coating approach. The GO membrane exhibited anumber of advantages over existing membranes. First, the GO membraneuses graphite as an inexpensive raw material, significantly lowering themembrane fabrication cost. Second, the synthesis procedure for both GOnanosheets and GO membrane is simple and scalable, thus providingtechnical readiness for scaling up the membrane production. In thepresent stage, the synthesized GO membrane had very high rejection of anorganic dye with a molecular weight of around 500 Daltons. Water flux ofthe GO membrane was about 4-10 times higher than that of most currentlycommercially available NF membranes. The facile synthesis of a GOmembrane exploiting the ideal properties of inexpensive GO materialsoffers a myriad of opportunities to modify its physicochemicalproperties, potentially making the GO membrane a next-generation,cost-effective, and sustainable alternative to the long-existingthin-film composite polyamide membranes for water separationapplications.

FIG. 3A illustrates a graph 300A of a zeta potential measurement of theGO nanosheets, in accordance with embodiments of the present disclosure,whereas FIG. 3B illustrates a graph 300B of X-ray photoelectronspectroscopy (XPS) data related to the GO nanosheets, in accordance withembodiments of the present disclosure.

FIG. 3C illustrates an atomic force microscopy (AFM) image 300C of theGO nanosheets, in accordance with embodiments of the present disclosure,whereas FIG. 3D illustrates a graph 300D of AFM height profiles of theGO nanosheets, in accordance with embodiments of the present disclosure.

Concerning the synthesis of GO nanosheets, GO nanosheets were preparedfrom graphite using a modified Hummers method. Flake graphite wasoxidized in a mixture of KMnO₄, H₂SO₄, and NaNO₃, then the resultingpasty GO was diluted and washed through cycles of filtration,centrifugation, and resuspension. The washed GO suspension wassubsequently ultrasonicated to exfoliate GO particles into GO nanosheetsand centrifuged at high speed to remove unexfoliated graphite residues.The resulting yellowish/light brown solution was the final GO nanosheetsuspension. This color indicated that the carbon lattice structure wasdistorted by the added oxygenated functional groups. The produced GOnanosheets were very hydrophilic and stayed suspended in water formonths without a sign of aggregation or deposition.

Concerning the characterization of GO nanosheets, a series ofcharacterization experiments were performed to understand the uniqueshape, functionality, and other physicochemical properties of GOnanosheets. These experiments included calculations related tozeta-potential analyzer for charge, Raman spectroscopy for G/D ratio,Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectronspectroscopy (XPS) for functional groups, X-ray diffraction (XRD) forcrystalline structure, and atomic force microscopy (AFM), SEM, andtransmission electron microscopy (TEM) for size and shape, as discussedbelow with reference to FIGS. 3A-3D.

During these experiments, the zeta potential measurement (see FIG. 3A)revealed that the GO nanosheets were negatively charged over a wide pHrange. The XPS data (see FIG. 3B) showed that about 60% of carbon wasnot oxidized, 32% had C—O bond (representing hydroxyl and epoxidegroups), and 7% had —COOH bond. Moreover, it is evident from the AFMimage (see FIG. 3C) that the lateral sizes of GO nanosheets variedbetween 100 and 5000 nm. The depth profiles (see FIG. 3D) obtained byanalysis of the AFM image demonstrated that the heights of the GOnanosheets were in the range of 1-2 nm, indicating that the GOnanosheets contained either single or double layers of carbon lattice.These characterization techniques were used to study the effects ofexperimental conditions (e.g., oxidation time, sonication strength andduration) on the properties of GO nanosheets.

FIG. 4A illustrates a covalently bonded GO synthesis strategy 400A, inaccordance with embodiments of the present disclosure, whereas FIG. 4Billustrates an electrostatically bonded GO synthesis strategy 400B, inaccordance with embodiments of the present disclosure.

Concerning membrane synthesis strategies, two readily scalable LbLsynthesis strategies are proposed to create GO membranes with a varietyof properties. As shown in FIG. 4A, the first strategy is to usecross-linkers to covalently bond the stacked GO nanosheets. The covalentbonding provides the stacked GO layers with the necessary stability toovercome their inherent tendency to disperse in water and also fine-tunethe charge, functionality, and spacing of the GO nanosheets. The secondstrategy (see FIG. 4B) is to assemble the oppositely charged GOnanosheets and polyelectrolytes to create a stacked membrane bonded byelectrostatic forces. Compared with the covalently bonded GO membrane,the electrostatically bonded GO membrane has highly charged surfaces,conveniently adjusted functionality (by varying polyelectrolytes withdifferent functionalities, charge density, and morphology), and possiblein-situ regenerability for fouling control.

FIG. 5A illustrates a quartz crystal microbalance with dissipation(QCM-D) graph 500A of the covalently bonded graphene oxide synthesisstrategy of FIG. 4A, in accordance with embodiments of the presentdisclosure, whereas FIG. 5B illustrates a quartz crystal microbalancewith dissipation (QCM-D) graph 500B of the electrostatically bondedgraphene oxide synthesis strategy of FIG. 4B, in accordance withembodiments of the present disclosure.

The feasibility of using the proposed strategies discussed above withreference to FIGS. 4A and 4B to synthesize GO membranes on a porouspolysulfone (PSI) support was examined. Quartz crystal microbalance withdissipation (QCM-D) was used to monitor the formation of thecovalently/electrostatically bonded GO membrane on a PSf-coated sensor.The QCM-D is very sensitive with a detection limit of 2 ng/cm². In thetest using the first synthesis strategy, 1,3,5-benzenetricarbonyltrichloride (TMC) was used as a cross-linker. The QCM-D results shown inFIG. 5A indicate that covalently bonded GO layers were successfullydeposited on the PSf support. FIG. 5B shows the successful synthesis ofan electrostatically bonded GO membrane by the alternate deposition ofnegatively charged GO and positively charged poly-L-lysine (PLL).

The membrane synthesis protocols were optimized to tune the propertiesof the GO membranes. Specifically, for the covalent bonding strategy,the size of GO nanochannels was adjusted by using cross-linkers withdifferent molecular weights and morphologies, such as monomers (e.g.,TMC, ethylenediamine) and polymers (e.g., polyethyleneimine, or PEI, andpoly(allylamine hydrochloride), or PAH, with different molecularweights). For the electrostatically bonding strategy, the structure,charge, and functionality of the GO membrane was tuned by usingpolyelectrolytes (e.g., PLL, PEI, PAH) with various sizes, morphologies,and charge densities.

FIG. 6A illustrates a graph 600A of water flux of covalently bondedgraphene oxide membranes, in accordance with embodiments of the presentdisclosure. FIG. 6B illustrates a graph 600B of the rejection rate of afirst chemical compound (first contaminant) when covalently bondedgraphene oxide membranes are used, in accordance with embodiments of thepresent disclosure, whereas FIG. 6C illustrates a graph 600C of therejection rate of a second chemical compound (second contaminant) whencovalently bonded graphene oxide membranes, in accordance withembodiments of the present disclosure.

Although simulation studies and water vapor experiments have indicatedthat GO membranes hold great potential for achieving extremely fastwater flux, to our best knowledge, this phenomenon has not beenexperimentally proven in aqueous phase applications. Therefore, theexemplary embodiments of the present disclosure develop the theoreticalbasis for water flow within the GO membrane and, subsequently, tooptimize membrane properties for improved water permeability.

The water flux of covalently bonded GO membranes was tested. As shown inFIG. 6A, water flux of membranes with 5 to 25 GO layers was in the rangeof 130-280 LMH/MPa, 5 to 10 times that of most existing polymericmembranes with a similar separation capability. It is interesting torecognize an apparent lack of correlation between water flux and thenumber of GO layers, indicating that the water resistance of GO coatingmay not be linearly correlated with the total thickness of the GO layersof a GO membrane. Note that this observation is consistent with the fluxbehavior of CNT membranes, whose water permeability is not significantlyaffected by the membrane thickness.

To understand the underlying mechanisms for water transport in GOmembranes, the effects of interlayer spacing, charge, and functionalityon water flux in covalently/electrostatically bonded GO membranes needsto be considered.

The following discussion provides a clear insight into the mechanismsfor the removal of different contaminants by GO membranes. Suchknowledge aids to rationally and systematically optimize the membraneselectivity. As plotted in FIG. 6B, the results of rejection tests usingcovalently bonded GO membranes show that membranes made of 5 to 25 GOlayers achieved 46-66% rejection of methylene-blue (MB) and 93-95%rejection of rhodamine-WT (R-WT). The higher rejection rate for R-WTcould be attributed to both size exclusion and charge effects. Forexample, R-WT has a higher molecular weight than MB, and both R-WT andthe GO membrane are negatively charged while MB is positively charged.

To better understand the influence of charge on the separationperformance of a GO membrane, an investigation was conducted regardingthe rejection of NaCl and Na₂SO₄ at different solution concentrations.As shown in FIG. 6C, the rejection decreases significantly as ionicstrength (i.e., concentration) increases. Note that the Debye lengthalso decreases with increasing ionic strength (e.g., 31 nm for 0.1 mMNaCl, 3 nm for 10 mM NaCl). The trends demonstrated that, as the Debyelength decreases, the electrostatic repulsion between ions and thecharged membrane decreases due to the suppression of electrostaticdouble layers, thereby causing the rejection rate to drop. Therefore,charge effects could significantly contribute to the separationmechanisms of a GO membrane. Note also that the rejection rate of the GOmembrane is comparable to that of a CNT membrane with sub-2-nm sizedpores, indicating that the spacing between GO nanosheets are around orless than 2 nm.

FIG. 7 illustrates a cross-sectional image 700 of the polyacrylonitrile(PAN) support, with front and back views, in accordance with embodimentsof the present disclosure.

Regarding the membrane support preparation, the membrane supportsubstrate was made of polyacrylonitrile (PAN) through phase inversionand partial hydrolysis. First, a PAN solution was prepared by dissolving18 g PAN (Mw≈150,000) and 2 g LiCl in 80 g N,N-dimethylformamide (DMF)(≥99.8%) at 60° C. All the chemicals were obtained from Sigma-Aldrich(St. Louis, Mo.). After cooling to room temperature, the PAN solutionwas stored overnight in a vacuum desiccator. Next, the PAN solution wascast on a clean glass plate using an aluminum casting rod with a gateheight of 125 μm. Then, the glass plate along with the cast PAN film wasimmediately soaked in a DI water bath for 10 min, during which phaseinversion took place to form the PAN support, which finally underwentpartial hydrolysis in 1.5 M NaOH for 1.5 h at 45° C. and was thoroughlyrinsed with DI water.

Regarding the GO membrane synthesis, the GO membrane was synthesized viathe LbL assembly of GO and PAH on the hydrolyzed PAN (hPAN) supportsubstrate. The GO solution (1 g/L, pH 4) was prepared using the modifiedHummers method. The PAH solution was prepared by dissolving 1 g/L PAH(Sigma-Aldrich, St. Louis, Mo.) in DI water and the pH was adjusted from4.6 to 4 using HCl and NaOH solutions. To synthesize the GO membrane, atypical assembly cycle involved soaking the hPAN support in the PAHsolution for 30 min and then in the GO solution for another 30 min,thereby adding one GO-PAH bilayer onto each side of the hPAN support.Repeating a prescribed number of such soaking cycles led to a GOmembrane with a desired number of GO-PAH bilayers on each side of thehPAN support. The GO membrane was thoroughly rinsed with DI waterbetween successive soaking treatments during the synthesis.

Regarding the membrane characterization and performance evaluation,membranes were characterized using various techniques and tested underhydraulic pressure, as well as in FO and PRO modes.

Regarding the quantification of the LbL assembly, quartz crystalmicrobalance with dissipation (QCM-D) (E-4, Biolin Scientific, LinthicumHeights, Md.) was used to monitor the process of assembling GO-PAHbilayers on an hPAN film. In summary, a QCM-D gold sensor (14 mm indiameter) was coated with a PAN film, hydrolyzed in 1.5 M NaOH solution,and mounted in a QCM-D chamber, along with a control bare sensor mountedin another chamber. Both sensors were successively exposed to PAH (1g/L, pH 4) and GO (1 g/L, pH 4) solutions to mimic the LbL assembly of aGO membrane. The mass of GO or PAH deposited on a sensor was quantifiedby monitoring and model-fitting the changes in frequency anddissipation, respectively, of the sensor against time using Q-Toolsoftware (Biolin Scientific, Linthicum Heights, Md.).

Regarding the quantification of charge density and partitioncoefficient, QCM-D can be effectively used to characterize the chargedensity of a thin film using, for example, CsCl as a probing species.Therefore, QCM-D was used to measure the charge densities of the PAN andhPAN supports, as well as the GO membrane. Note that, after the chargeprobing of the GO membrane, the sensors were exposed to 1 M MgCl₂, 0.25M trisodium citrate (TSC), and 1 M sucrose solutions, respectively, tostudy the partition of each type of draw solute into the GO membrane.

Regarding preparation of the membrane support, due to the convenience inmanipulating its structure and functional groups, PAN was selected tofabricate the membrane support via phase inversion. FIG. 7 shows thecross-section of the PAN support, which was relatively thin (˜60 μm) andcontained finger-like structures with low tortuosity. There were denseskin layers on both sides of the support, with one side (referred to asthe front side, see middle section of FIG. 7) even denser and smootherthan the other side (referred to as the back side, see right section ofFIG. 7). Such dense skin layers are ideal for effectively forming adense barrier layer with much less imperfection on each side of thesupport.

FIG. 8 illustrates a graph 800 of the Fourier Transform InfraredSpectroscopy (FTIR) spectra of the membrane, in accordance withembodiments of the present disclosure.

PAN was partially hydrolyzed to turn into hPAN, such that a portion ofnitrile functional groups (—C∂N) were converted to carboxylatefunctional groups (—COO⁻), which were required for the membrane supportto securely attach the first PAH layer. As revealed by the FTIR spectrain FIG. 8, both PAN and hPAN spectra have a signature peak at 1450 cm⁻¹for nitrile, while partial hydrolysis led to a small new peak in thehPAN spectra at 1560 cm⁻¹, confirming the formation of carboxylatefunctional groups.

FIG. 9 illustrates a graph 900 of elemental ratios of membrane supportsand GO membranes by XPS analysis, in accordance with embodiments of thepresent disclosure.

The replacement of nitrile by carboxylate was also verified by thedecreased N/C ratio and the increased O/C ratio. In addition, the O/Cratio for the front side (i.e., the side with smaller pores) of hPAN ismuch higher than that on its back side, possibly because the degree ofhydrolysis on the exposed surface was much larger than inside thesupport and because the penetration depth of XPS on the back side washigher due to the looser structure there. Note that the partialhydrolysis of PAN did not cause observable changes in thecross-sectional structure but slightly narrowed the surface pores of themembrane support, as shown in the boxed areas of FIG. 7 (middle andright sections). The relatively small pore sizes and smooth surface madehPAN an ideal substrate for the LbL assembly.

FIG. 10 illustrates a graph 1000 of charge density of membrane supportsby QCM-D, in accordance with embodiments of the present disclosure.

Concerning the properties of the membrane support, GO, and PAH, thecharges of PAN and hPAN were probed by Cs⁺ in QCM-D experiments. Asdemonstrated in FIG. 10, the PAN support did not carry any detectablecharge at any studied pH. In contrast, the hPAN support containedsignificant negative charges at pHs 7 and 10 but very low charges at pH4, confirming the successful conversion of nitrile to carboxylatefunctional groups, which have a pKa of ˜4. Despite the low chargedensity at pH 4, the first PAH layer was successfully assembled on thehPAN support, indicating that interactions (e.g., hydrophobic force,hydrogen bonding) besides electrostatic interaction also played a rolein the deposition of the first PAH layer.

FIG. 11 illustrates a graph 1100 of zeta potentials of GO and PAH atdifferent pHs, in accordance with embodiments of the present disclosure.

The charge properties of GO and PAH were analyzed using zeta potentialmeasurement to evaluate the feasibility of the LbL assembly of GO-PAHbilayers via electrostatic interaction. As shown in FIG. 11, GO and PAHwere able to remain positively and negatively charged, respectively,over a wide pH range of 2 to 10, thereby ensuring the stability of theelectrostatically assembled GO-PAH bilayers and eventually the GOmembrane.

FIG. 12 illustrates a schematic diagram 1200 of an LbL assembly of a GOmembrane by alternately soaking an hPAN support substrate in 1 g/L PAH(pH 4) solution and 1 g/L GO solution (pH 4), in accordance withembodiments of the present disclosure.

Concerning the synthesis of the GO membrane, the process of LbL assemblyof a GO membrane is schematically illustrated in FIG. 12. The hPANsupport was first immersed in the PAH solution to attach positivelycharged PAH, and then in GO solution to deposit negatively charged GO ontop of PAH, thus completing the assembly of the first GO-PAH bilayer oneach side of the hPAN support. Such a deposition cycle was repeated toassemble a desired number of GO-PAH bilayers. Note that the pHs of bothPAH and GO solutions were kept at 4, close to the natural values of theas-prepared solutions. Therefore, significant amount of acid or base wasnot needed to adjust the solution pH and hence the solution ionicstrength was kept to a minimum, thereby avoiding the formation of aloosely packed membrane structure due to the otherwise hydration ofpolyelectrolytes and also preventing GO nanosheets from aggregation dueto the charge screening effect.

FIG. 13 illustrates SEM images 1300 of the GO membranes made ofdifferent numbers of GO-PAH bilayers, in accordance with embodiments ofthe present disclosure.

Concerning the characterization of the GO membrane, the SEM images inFIG. 12 clearly show that the two sides of GO membranes were rougherthan those of the original hPAN support, respectively, indicating asuccessful assembly of multiple GO-PAH bilayers. Also note that thereexisted a significant difference in the surface morphology of the twosides of the original hPAN support but such a difference was reduced asthe number of GO-PAH bilayers increased, indicating that a highersurface coverage by GO-PAH was achieved on both sides of the hPANsupport.

Additional evidence is available for the successful assembly of GO-PAHbilayers by the LbL procedure. For example, FIG. 9 shows that the O/Cratio gradually decreases with the increasing number of GO-PAH bilayers.In particular, the O/C ratio for both front and back sides of the10-bilayer GO membrane reaches almost the same value of 0.24, indicatingthat the two surfaces attained the same level of coverage by GO-PAH. TheFUR spectra in FIG. 8 also reveal that the intensity of the carboxylategroups at 1560 cm⁻¹ increased as more GO-PAH bilayers were assembled.

FIG. 14 illustrates a graph 1400 of cumulative masses of GO and PAHduring the LbL assembly of a GO-PAH film on an hPAN-coated QCM-D sensor,in accordance with embodiments of the present disclosure.

Regarding the composition and the thickness of the GO membrane, QCM-Dwas employed to monitor the LbL assembly of GO-PAH bilayers so as toquantify the composition and thickness of the GO membrane. The rawfrequency and dissipation data, as well as the derived mass ratios ofPAH to GO were used. FIG. 14 clearly shows that the masses of GO and PAHboth increased steadily with the increasing number of bilayers, provingthe successful assembly of multiple GO-PAH bilayers. It is also observedin FIG. 14 that the mass of GO was consistently greater than (2 to 5times) that of PAH after any deposition cycle, most likely due to thecombined effects of the higher charge density (and thus lowermass/charge ratio) of PAH and the larger lateral dimensions (and thushigher mass/charge ratio) of GO.

The quantified mass of GO and PAH enables the estimation of the totalGO-PAH thickness of a GO membrane. For example, the 10-bilayer GOmembrane had a total deposited mass of 18.2 μg/cm² on the hPAN-coatedsensor. Assuming a GO membrane density of 1.1 g/cm³, it is estimatedthat the total GO-PAH thickness on each side of the hPAN support is ˜165nm and hence on average a single GO-PAH bilayer is ˜16.5 nm thick, muchmore than that (˜1 nm) of a pure GO layer in previously reported GOmembranes, suggesting that multiple GO layers were deposited during eachGO-PAH deposition cycle.

The high rejection of sucrose by the GO membrane indicates that the GOchannel size (i.e., the clear inter-GO-layer spacing, h) was −1 nm,further supporting the existence of multiple GO layers within one GO-PAHbilayer. Since the thickness of a single GO nanosheet, do, is ˜0.3 nm, atypical GO layer in the present GO membrane should have an overallthickness of d=h+d₀=˜1.3 nm, keeping in mind that PAH might besandwiched as a spacer between GO nanosheets.

Therefore, a total of 16.5/1.3≈13 GO layers may exist in one GO-PAHbilayer. In fact, deposition of multiple GO layers during one assemblycycle is quite reasonable because, compared with PAH, GO has a lowcharge density and hence multiple GO layers were needed to compensateall charges on PAH. Furthermore, as most charges on GO are located alongits edges, it is possible that multiple GO nanosheets, since they mightnot be deposited perfectly flat but at an angle, partially overlapped asthey electrostatically edge-connected themselves to PAH, thereby formingmultiple GO layers during one deposition cycle. FIG. 13 illustrates thestructure of a GO membrane where multiple GO-layers (and thus multipleinter-GO channels) exist in each GO-PAH bilayer.

FIG. 15 illustrates a graph 1500 of pure water permeability underhydraulic pressure, in accordance with embodiments of the presentdisclosure.

Concerning water transport in GO membranes, the water flux of the GOmembrane was measured in a hydraulically pressurized membrane system. Asshown in FIG. 15, the water permeability (i.e., water flux normalized bytransmembrane pressure) of the GO membranes ranged from 2.1 to 5.8LMH/atm and was much lower than that of the PAN (88.4 LMH/atm) and hPANsupport (19.0 LMH/atm), indicating that the existence of GO-PAH bilayersled to significant hydraulic resistance and hence lowered the waterflux.

The 10-bilayer GO membrane is taken as an example to estimate thevelocity of water transport within a GO membrane. Assuming on average aGO lateral dimension of 500 nm, clear inter-GO-layer spacing of 1 nm,and single GO-PAH bilayer thickness of 16.5 nm, the water permeability(5.8 LMH/atm) of the GO membrane can be converted to a water transportvelocity of 4.8×10⁻⁴ m/s under a 1-atm transmembrane pressure. Thisestimated velocity turns out to be two orders of magnitude higher thanthe velocity (8.4×10⁻⁷ m/s) of water flowing between two hypotheticalparallel plates, as predicted by the plate-Poiseuille equation. Hence,an experimental verification of a fast water transport through GOchannels in an electrostatically assembled GO membrane is presented.Upon knowing that the sandwiching of certain polymers (e.g., PAH)between GO layers may not necessarily reduce water transport velocity ina layered GO membrane, researchers are encouraged to explore thesynthesis of highly tunable GO membranes by using carefully selectedpolymeric spacers that have exceptional properties.

At the current stage, the water permeability of the GO membrane is oneorder of magnitude higher than that (0.36±0.11 LMH/atm) of thecommercial HTI membrane, as compared in FIG. 15. Note that the GOmembrane permeability can be further improved by increasing GO porosityand decreasing its tortuosity, both of which can be achieved by, forexample, optimizing GO lateral dimension, creating vertically aligned GOnanosheets (i.e., generating straight-through GO channels), and varyingdeposition conditions.

FIG. 16 illustrates a graph 1600 of water flux in FO and PRO modes with1M sucrose, in accordance with embodiments of the present disclosure.

Concerning water flux and GO membrane in FO and PRO, the water flux ofthe GO membrane was tested in FO and PRO modes using DI water as feedsolution and using 1 M sucrose, 1 M MgCl₂, and 0.25 M TSC as drawsolutions, respectively. FIG. 16 shows that, when sucrose was used asdraw solute, the GO membrane flux was about 3 to 4 times that of the HTImembrane in FO and PRO modes, respectively. Note that water flux of theGO membrane in PRO mode was more than twice that in FO mode, indicatingthe existence of significant internal concentration polarization (ICP)in FO mode. The structural integrity and separation capability of theGO-PAH bilayers deposited on the back side of the hPAN support are lessthan that on the front side. This hypothesis is consistent with theobservation in FIG. 15 that as the number of GO-PAH bilayers increased,the flux difference between FO and PRO modes decreased in general.

FIG. 17 illustrates a graph 1700 of water flux in FO and PRO modes with1M MgCl₂, in accordance with embodiments of the present disclosure,whereas FIG. 18 illustrates a graph 1800 of water flux in FO and PROmodes with 0.25 M TSC, in accordance with embodiments of the presentdisclosure.

It is observed in FIGS. 17 and 18 that water fluxes of a GO membrane inFO and PRO modes were very similar when using MgCl₂ or TSC as drawsolute, indicating that internal concentration polarization (ICP) wouldsimultaneously exist or be absent in FO and PRO modes. The ion (MgCl₂and TSC) transport flux of a GO membrane was relatively high, indicatingthat the GO-PAH bilayers unlikely created a barrier that blocked theions from entering the hPAN support. Therefore, it is reasonable tobelieve that ICP was present in both FO and PRO modes for the case ofMgCl₂ or TSC as draw solute. In fact, it has been demonstrated thatpolyelectrolyte films can significantly hydrate and expand theirthickness under high ionic strength. As a result, regardless of thedirection that a GO membrane was placed in the test system (i.e., use ofthe membrane in FO vs. PRO mode), the GO-PAH bilayers in contact withthe high-concentration draw solution would hydrate and lead to a loosestructure, which allowed ions to transport and hence resulted in ICP inthe membrane support.

FIG. 19 illustrates a graph 1900 of sucrose permeation flux, inaccordance with embodiments of the present disclosure.

Concerning the mechanisms of solute transport in GO membranes, thesolute flux in FO, as well as in PRO mode was measured. FIG. 19 showsthat the sucrose flux of a GO membrane was more than 7 times that of anHTI membrane. However, there seems no conclusive correlation between thenumber of bilayers and the solute flux of a GO membrane. Note that thesucrose flux of a GO membrane in FO mode was consistently lower thanthat in PRO mode. This is because typically the ICP in FO mode causedthe dilution of draw solution in the membrane support, therebyminimizing the passage of solutes through the membrane.

Information on solute rejection of the GO membrane can be used toestimate its pore cutoff size. The 10-bilayer GO membrane exhibited muchhigher rejection of sucrose (99%) than that of MgCl₂ (78%) and TSC(90%), indicating that the channel cutoff size (i.e., the inter-GO-layerspacing) of the GO membrane was close to the hydrated diameter ofsucrose (˜1 nm). The relatively low rejection of ionic species can bemost likely attributed to the hydration effect of the GO-PAH film underhigh ionic strength. Therefore, the GO membrane at the current stage maynot be directly applicable for desalination, because without covalentcross-linking, the GO membrane would probably swell under high ionicstrength and thus considerably lose its solute rejection capability.Nevertheless, the present GO membrane can be well suited for manyimportant applications such as FO-based emergency water supply systemsthat use sugary draw solutions as well as water purification andwastewater reuse that do not mandate high ionic strength conditions.

In order to determine the mechanisms controlling the solute transport inthe GO membrane, the partition coefficients and diffusion coefficientswere characterized for the three draw solutes. Partition coefficients ofthe three solutes were between 2.4 and 3.0 (so they do not differsignificantly), indicating that partitioning of these solutes into theGO membrane was neither affected by the size or charge of the specificsolute nor a governing factor for the huge difference in the permeationof these solutes.

FIG. 20 illustrates a graph 2000 of diffusion coefficients of drawsolutes for the 10-bilayer GO membrane, compared with those for water,in accordance with embodiments of the present disclosure.

As plotted in FIG. 20, the diffusion coefficients of MgCl₂ (4.2×10⁻⁶cm²/s), TSC (2.6×10⁻⁶ cm²/s), and sucrose (4.3 to 5.2×10⁻⁶ cm²/s) aresimilar in bulk water. Their diffusion coefficients for the GO membrane,however, are 3 to 5 orders of magnitude lower. The hindering effect isthe most pronounced for sucrose, with the lowest diffusion coefficientof 4.1×10⁻¹¹ cm²/s. The ring structure in sucrose may have stronginteractions with the carbon rings in GO, thereby increasing thehindrance by friction and decreasing the diffusion of sucrose. The muchlower hindering effects for the two ionic species (MgCl₂ and TSC) againcan be attributed to the hydration of GO-PAH films under high ionicstrength.

A closer comparison of the two ionic species (MgCl₂ and TSC) helpsidentify the most possible solute transport path in the GO membrane.Since TSC is composed of one C₆H₅O₇ ³⁻ and three Nat, the diffusion ofC₆H₅O₇ ³⁻ should be faster than TSC and thus more than 25 times that ofsucrose, although the hydrated radius of C₆H₅O₇ ³⁻ is even slightlyhigher than that of sucrose. The increase in the transport rate ofnegative ions indicates that the dominant path for solute transport wasnegatively charged, since a positively charged path would tend to adsorbnegative ions onto its surface and thereby increase the ion-surfacefriction and consequently decrease the diffusion rate. This indicationis further reinforced by the fact that the diffusion coefficient ofMgCl₂ (controlled by Mg²⁺) is only one fourth that of TSC, althoughMgCl₂ diffuses faster than TSC in bulk water. Therefore, the dominanttransport path in the GO membrane should be that formed by thenegatively charged GO nanosheets instead of positively charged PAHpolymers. This conclusion is consistent with the foregoing observationthat the existence of PAH did not interfere with the fast watertransport in GO channels, indicating that most likely PAH connected GOnanosheets only at their edges while leaving their surface areas largelyuntouched.

In summary, the exemplary embodiments of the present disclosure relateto a unique 2D structure of GO that makes it ideal for synthesizing anew class of membrane by stacking GO nanosheets via a layer-by-layer(LbL) assembly technique, which is relatively cost-effective andenvironmentally friendly because all fabrication steps can be performedin aqueous solutions while traditional membrane synthesis procedures(e.g., interfacial polymerization) often involve complex chemicalreactions and use organic solvents.

In the exemplary embodiments of the present disclosure, potential use oflayered graphene oxide (GO) membrane in forward osmosis (FO) andpressure retarded osmosis (PRO) processes is contemplated. As anenergy-inexpensive alternative to the conventional pressure-drivenmembrane processes, the FO/PRO membrane technology has experienced anaccelerated development over the past decade. In contrast to other typesof water purification membranes, FO/PRO membranes must have a relativelythin, hydrophilic support in order to reduce internal concentrationpolarization, which is caused by the hindered solute transport withinthe support layer and can significantly reduce membrane flux andaggravate membrane fouling. To date, the existing commercial FO/PROmembranes can be categorized into cellulose-based membranes andthin-film composite (TFC) membranes. Synthesized via phase separation,cellulose-based membranes have excellent antifouling properties butexhibit relatively low water flux and high salt passage and only workwithin a narrow pH range. In comparison, TFC membranes show excellentsalt rejection at the cost of low pure water flux due to the thick,dense membrane support. Therefore, development of high-performanceFO/PRO membranes has been a major task in the journey of achieving thefull benefit of such a sustainable technology.

In the exemplary embodiments of the present disclosure, it is proposedto electrostatically bond layered GO nanosheets to form a stable GOmembrane and test its suitability for FO/PRO processes. The LbLtechnique was employed to assemble oppositely charged GO andpoly(allylamine hydrochloride) (PAH) layers on both sides of a chargedsupport substrate. The synthesized GO membrane was characterized using aseries of techniques to confirm the successful assembly of multipleGO-PAH bilayers, quantify their composition and thickness, andunderstand the structure and charge properties of the GO membrane. TheGO membrane was then tested in cross-flow pressurized and also in FO/PROmembrane systems in order to understand the mechanisms of the transportof water and solutes within the GO membrane, using a commerciallyavailable FO membrane as a baseline.

The facile/scalable synthesis and surface modification, exceptionalproperties, and fundamental mechanisms of the novelgraphene-oxide-enabled membranes may transform the development of a nextgeneration of high-performance, energy-efficient, low-cost membranes,which also have various important applications including: (1)point-of-use water purification for military operation missions and forhumanitarian relief to disaster-ridden and impoverished areas; (2)on-site treatment of hydrofracking flowback water; (3) renewable energyproduction; and (4) drug delivery and artificial organ development.Introduction of the new membrane technology will add a significantdriving force to the economy. Therefore, potential environmental,economic, and social benefits can be enormous.

Persons skilled in the art will understand that the devices and methodsspecifically described herein and illustrated in the accompanyingdrawings are non-limiting exemplary embodiments. The featuresillustrated or described in connection with one exemplary embodiment maybe combined with the features of other embodiments. Such modificationsand variations are intended to be included within the scope of thepresent disclosure.

The foregoing examples illustrate various aspects of the presentdisclosure and practice of the methods of the present disclosure. Theexamples are not intended to provide an exhaustive description of themany different embodiments of the present disclosure. Thus, although theforegoing present disclosure has been described in some detail by way ofillustration and example for purposes of clarity and understanding,those of ordinary skill in the art will realize readily that manychanges and modifications may be made thereto without departing form thespirit or scope of the present disclosure.

While several embodiments of the disclosure have been shown in thedrawings and described in detail hereinabove, it is not intended thatthe disclosure be limited thereto, as it is intended that the disclosurebe as broad in scope as the art will allow. Therefore, the abovedescription and appended drawings should not be construed as limiting,but merely as exemplifications of particular embodiments. Those skilledin the art will envision other modifications within the scope and spiritof the claims appended hereto.

1.-9. (canceled)
 10. A filtration apparatus comprising: a supportsubstrate; and a graphene oxide membrane disposed on the supportsubstrate, the graphene oxide membrane including a plurality of grapheneoxide sheets, each of the graphene oxide sheets covalently coupled to anadjacent graphene oxide sheet via a cross-linker.
 11. The filtrationapparatus of claim 10, wherein the graphene oxide membrane comprises 5to 25 layers of graphene oxide sheets.
 12. The filtration apparatus ofclaim 10, wherein the graphene oxide membrane has a rejection rate of93-95% of rhodamine-WT.
 13. The filtration apparatus of claim 10,wherein the graphene oxide membrane has a rejection rate of about 90% ofNa₂SO₄, as measured with a 0.1 mM Na₂SO₄ solution.
 14. The filtrationapparatus of claim 10, wherein the support substrate comprisespolysulfone.
 15. The filtration apparatus of claim 10, wherein thesupport substrate comprises polyacrylonitrile.
 16. The filtrationapparatus of claim 10, wherein the cross-linker is a monomer.
 17. Thefiltration apparatus of claim 16, wherein the monomer isethylenediamine.
 18. The filtration apparatus of claim 10, wherein thecross-linker is a polymer.
 19. A filtration apparatus comprising: asupport substrate; and a graphene oxide membrane disposed on the supportsubstrate, the graphene oxide membrane including 5 to 25 layers ofgraphene oxide sheets, each of the graphene oxide sheets covalentlycoupled to an adjacent graphene oxide sheet via a cross-linker, whereinthe graphene oxide membrane has a rejection rate of about 90% of Na₂SO₄,as measured with a 0.1 mM Na₂SO₄ solution.
 20. The filtration apparatusof claim 19, wherein the graphene oxide membrane has a rejection rate of93-95% of rhodamine-WT.
 21. The filtration apparatus of claim 19,wherein the support substrate comprises polysulfone.
 22. The filtrationapparatus of claim 19, wherein the support substrate comprisespolyacrylonitrile.
 23. The filtration apparatus of claim 19, wherein thecross-linker is a monomer.
 24. The filtration apparatus of claim 23,wherein the monomer is ethylenediamine.
 25. A filtration apparatuscomprising: a support substrate; and a graphene oxide membrane disposedon the support substrate, the graphene oxide membrane including 5 to 25layers of graphene oxide sheets, each of the graphene oxide sheetscovalently coupled to an adjacent graphene oxide sheet via a monomercross-linker.
 26. The filtration apparatus of claim 25, wherein themonomer cross-linker is ethylenediamine.
 27. The filtration apparatus ofclaim 25, wherein the graphene oxide membrane has a rejection rate of93-95% of rhodamine-WT.
 28. The filtration apparatus of claim 25,wherein the support substrate comprises polysulfone.
 29. The filtrationapparatus of claim 25, wherein the support substrate comprisespolyacrylonitrile.